1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a color cathode ray tube capable of ensuring high-luminance, high-contrast display.
2. Description of the Related Art
In general, a conventional color cathode ray tube comprises a transparent faceplate having an effective display region for displaying an image, a phosphor screen covering the inside of the effective display region of the faceplate, and a funnel whose front end portion is bonded to the rear side of the faceplate. The faceplate and the funnel constitute an envelope, which serves as a shell of the cathode ray tube. The rear end portion of the funnel forms a tapered neck, which contains an electron gun. The electron gun emits an electron beam on that portion of the phosphor screen which corresponds in position to the displayed image in a manner such that the phosphor screen is scanned with the electron beam.
The path of the electron beam emitted from the electron gun is controlled by means of a magnetic field generated by a deflecting yoke, which is arranged so as to surround the beam path. Thus, the electron beam is projected on a predetermined portion of the phosphor screen.
The brightness of the displayed image is one of important characteristics for the image quality of the a color cathode ray tube. Generally, the image brightness is evaluated by luminance and contrast characteristic.
The brightness and contrast of an image on a display screen visually recognized by an observer depends on the brightness of the surface of the display screen itself, as well as on the luminance of the displayed image. More specifically, the apparent brightness and contrast of the displayed image are settled depending on the sum total of the respective apparent brightness of reflected light from the display screen in a no-display state and the phosphor screen itself and the luminance of the displayed image from the phosphor screen.
The image display quality of the color cathode ray tube can be improved by improving the brightness and contrast of the displayed image.
It has been difficult, however, to improve the brightness and contrast concurrently using known techniques.
By improving the light transmittance of the faceplate by appropriately selecting its material, light emitted from the phosphor screen and projected on the front of the faceplate can be effectively utilized with high transmittance, and the displayed image can be brighter.
In the case where the faceplate of the high-transmittance material is used in this manner, the lightness of the nonluminous phosphor screen itself adds to the brightness of the display screen in the no-display state, in general. The contrast of an image, which is displayed as the phosphor screen is excited to glow, is settled depending on the correlation between the respective brightness of the displayed image and the display screen in the no-display state. The brighter the display screen in the no-display state, the lower the contrast characteristic of the displayed image. Since the color of the phosphor screen itself is based on white, in general, its lightness is very high. If the high-transmittance faceplate is used, therefore, the contrast of the displayed image tends to be lower.
Thus, in order to improve the luminance of the displayed image, according the conventional color cathode ray tube, a method is generally employed such that the driving voltage of the tube is raised to augment the energy of the electron beam, whereby the luminance of emission from the phosphor is enhanced.
Conventionally, the contrast characteristic is improved by a method in which the faceplate is formed of colored glass such that its light transmittance is not higher than 40%, for example, whereby the brightness of the display screen in the no-display state is lowered, or a method in which the phosphor is loaded with a pigment such that the phosphor screen itself is dark-colored.
If the electron beam energy is augmented, however, the power consumption of the whole color cathode ray tube increases, so that the economical efficiency is not very high.
If the faceplate is formed of colored glass such that its light transmittance is not higher than 40%, for example, the luminance of the displayed image lowers in proportion to the light transmittance of the faceplate, while the external light reflectance lowers in proportion to the square of the transmittance of the faceplate, so that the contrast characteristic of the displayed image is improved. If the light transmittance of the faceplate itself lowers, however, the transmittance of the light that is emitted from the phosphor screen and transmitted through the faceplate to form the image also lowers to 40% or less. As a result, the luminance of the displayed image is lowered considerably.
Recently, there has been provided a cathode ray tube with a "flattened" faceplate, which is shaped so that its thickness increases from its center toward the peripheral portion at the predetermined changing rate. In the color cathode ray tube of this type, the coefficient of absorption of the light that forms the displayed image is higher at the thickened peripheral portion of the faceplate, so that the luminance of the displayed image at the peripheral portion of the display screen greatly differs from the luminance at the central portion.
According to the color cathode ray tube using the "flattened" faceplate, making the thickness of the faceplate uniform throughout the display screen requires a special shape of a shadow mask in the tube and special methods for scanning electron beams and manufacturing the whole cathode ray tube. Thus, there are so many restrictions on manufacture and construction that it is very hard practically to make the faceplate uniform in thickness.
Possibly, the contrast characteristic may be improved by loading the phosphor with a pigment to darken the phosphor screen itself. Since the pigment is not a phosphor, however, the percentage of compositions that are not conducive to emission in the phosphor screen is so high that the emission efficiency of the phosphor layer itself is lowered. In consequence, the luminance of the displayed image is lowered.
Alternatively, the emission in the phosphor screen may be enhanced by increasing voltage for emitting the electron beam from the electron gun to augment the energy of the electron beam. In this case, however, the power consumption increases, and voltage for deflecting the high-energy electron beam must be increased, thus requiring an additional increase in the power consumption. After all, the power consumption of the whole color cathode ray tube is increased considerably.
In order to solve the above problems, a novel technique or method has been proposed in which a color filter layer is interposed between the inner surface of the faceplate and the phosphor screen. Recently, this method has come to public notice.
The color filter layer includes a plurality of color filters, which are opposed to phosphor dots or stripes that form differently colored pixels of the phosphor screen. Each filter transmits only light beams having the color of its corresponding pixel. Arranged in this manner, the color filter layer projects only those emitted light beams which correspond to the color of each pixel on the faceplate, and prevents external light from being reflected by the interface between the phosphor screen and the faceplate. Thus, the contrast characteristic is believed to be able to be effectively improved without lowering the chromaticity of the luminous spots of different colors on the display screen, that is, the color intensity of the emitted light beams, or the luminance of the display screen.
Since the color filter layer is disposed on the inner surface of the faceplate, however, its material is restricted to a heat-resisting transparent material, such as an inorganic pigment, that can stand the internal environment of the color cathode ray tube. Due to this restriction on the material, the color filter layer cannot always enjoy optimum filtering characteristics for satisfactory filter functions.
Among the luminous spots of different colors that constitute the image displayed on the display screen, red spots R have the greatest influence on the quality of reproduction of the image. The spectrum distribution of red light beams emitted from the red spots R or phosphors corresponding to red is very narrow and sharp, and the color intensity of the light beams emitted from the spots R is higher if the spectra are displayed without change.
In the case where the aforesaid color filter layer is used, however, the red spots R on the display screen are represented as light beams of a color that resemble the spectra of red light from the phosphors to some degree. In the existing circumstances, however, the emission spectra of the red spots involve great sub-bands that are attributable to the characteristics of the color filter layer. Inevitably, therefore, the red spots of the displayed image are recognized as low-intensity luminous spots by the observer. This problem on the color intensity is common to all colors including green (G) and blue (B) as well as red (R).
Thus, the prior art color cathode ray tube has a problem that satisfactory improvement of the luminance and contrast characteristic of the effective display region is incompatible with improvement of the lightness and color intensity of the luminous spots of the individual colors, including red, on the displayed image.